1. Field of the Invention
The present invention relates to a reinforced hose having an inner rubber layer, an outer rubber layer, and at least two reinforcing layers including an upper yarn layer and a lower yarn layer.
2. Description of the Related Art
Of this type reinforced hoses, a hose shown in FIG. 7 is heretofore known as a brake hose used in a vehicle. FIG. 7 is a sectional view showing important part of the brake hose 100 according to the related art. The brake hose 100 is formed in such a manner that a plurality of rubber and fiber yarn layers are laminated because the brake hose 100 needs to have high pressure resistance to brake oil pressure. That is, the brake hose 100 is formed as a laminate which has: an inner rubber layer 102 for forming a flow path 101 in which brake oil flows; a lower yarn layer 104; an intermediate rubber layer 106; an upper yarn layer 108; and an outer rubber layer 110,
In the brake hose 100, fluid pressure is transmitted from the inner rubber layer 102 to the lower yarn layer 104, the intermediate rubber layer 106 and the upper yarn layer 108 successively and transmitted to the outer rubber layer 110 finally. Constraint force which suppresses expansion against the pressure of pressure fluid is generated in the respective layers. On this occasion, large part of the constraint force depends on the lower yarn layer 104 and the upper yarn layer 108 because the respective rubber layers, that is, the inner rubber layer 102, the intermediate rubber layer 106 and the outer rubber layer 110 are so highly elastic that only small part of the constraint force is generated in the respective rubber layers. Therefore, to increase constraint force generated on the basis of reaction force caused by expansion of the lower yarn layer 104 and the upper yarn layer 108 is to improve expansion resistance of the brake hose 100, that is, to reduce the amount of volumetric expansion. To improve durability of the lower yarn layer 104 and the upper yarn layer 108 against repeated contraction and expansion is to improve durability of the brake hose 100 per se. However, fiber yarn having a high stretchability is excellent in durability but inferior in expansion resistance whereas fiber yarn having a low stretchability is excellent in expansion resistance but inferior in durability. That is, durability and expansion resistance are contrary to each other.
A technique using high-modulus polyester yarn (high-modulus PET yarn) in the lower yarn layer 104 to keep durability high and using inextensional vinylon yarn in the upper yarn layer 108 to reduce the amount of volumetric expansion is known as a technique for successfully combining durability and expansion resistance with each other (see Japanese Patent No. 2,692,480). A technique in which lower yarn constituted by fine fiber yarn is braided to form a thin and dense reinforcing layer to thereby attain improvement in durability and expansion resistance is also known (see Japanese Patent No. 3,003,769).
The demand for higher pressure resistance and higher expansion resistance, however, has increased recently with the advance of increase in pressure of a hydraulic device of a vehicle. sufficient characteristic has never been obtained by the aforementioned change in kind of yarn in the lower yarn layer 104 and the upper yarn layer 108.